Download 6. Neurotransmitters

Chemical Messengers
Neurotransmitters
Hormones
Supplemental
© 2013 Pearson Education,
Inc.
Chapter 18, Section 6
4/13/2013
Chemical Messengers:
for coordination of biological processes within an organism
 Terms and Definitions:
 neuron: a nerve cell.
 neurotransmitter: a chemical messenger between a neuron
and another target cell; neuron, muscle cell or cell of a
gland.
- the molecule acts over a short distance
(across a synapse, ~ 0.1 µm)
Eg, Acetylcholine
 hormone: a chemical messenger released by an endocrine
gland (secretary organ) into the bloodstream and
transported there to reach its target cell (its site of action).
over a long distance (~ 20cm).
Eg, Insulin
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Chapter 18, Section 6
Chemical Messengers
 Five Classes: :
 Cholinergic Messengers: Acetylcholine, and likes
 Amino acid Messengers: Glutamate, GABA, etc.
 Adrenergic Messengers: Monoamines
 Peptidergic Messengers: Insulin, Glucagon, etc.
 Steroid Messengers:
Androgen, Estrogen, etc.
 Messengers are also classified by how they work;
 activate enzymes.
 affect the synthesis of enzymes.
 affect the permeability of membranes.
 act directly or through a secondary messenger.
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Chapter 18, Section 6
Chemical Communication
 Many drugs used in medicine influence chemical
communication
 Antagonist: a molecule that blocks a natural receptor and
prevents its stimulation.
 Agonist: a molecule that competes with a natural
messenger for a receptor site; it binds to the receptor site
and elicits the same response as the natural messenger.
 A drug may decrease or increase the effective
concentration of messenger.
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Chapter 18, Section 6
 A neurotransmitter
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Chapter 18, Section 6
5
Acetylcholine (ACh): the main cholinergic messenger
the first neurotransmitter found (1914)
by Henry Dale & Otto Loewi (NP, 1936)
 communicate between the nervous system and the muscle.
 When stimulated, it releases into the synapse where it binds
to muscle cell receptors causing the muscles to contract.
O
CH3
CH3 -C-O-CH2 -CH2 +
-N-CH3
CH3
Acetylch oline (A Ch )
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Amine (Quaternary) as well as Esther
Chapter 18, Section 6
6
Acetylcholine
 Storage and release of acetylcholine (ACh).
 The nerve cells that bring messages contain ACh stored in
vesicles.
 The receptors on muscle neurons are called nicotinic
receptors because nicotine inhibits them.
 The message is initiated by calcium ions, Ca2+.
 When Ca2+ concentration becomes more than about 0.1
mM, the vesicles that contain ACh fuse with the
presynaptic membrane of nerve cells and empty ACh into
the synapse.
 ACh travels across the synapse and is absorbed on
specific receptor sites.
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Chapter 18, Section 6
Acetylcholine
 Action of the acetylcholine (cont’d)
 The presence of ACh on the postsynaptic receptor
triggers a conformational change in the receptor protein.
 This change opens an ion channel and allows ions to
cross membranes freely.
 Na+ ions have higher concentration outside the neuron
and pass into the cell.
 K+ ions have higher concentration inside the neuron and
leave the cell
 This change of Na+ and K+ ion concentrations is
translated into a nerve signal.
 After a few milliseconds, the ion channel closes.
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Chapter 18, Section 6
Acetylcholine in Action
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Chapter 18, Section 6
Acetylcholine
 Removal of ACh
 ACh is removed from the receptor site by hydrolysis
catalyzed by the enzyme acetylcholinesterase.
Acetylch olin O
CH3
es teras e
CH3 -C-O-CH2 -CH2 +
-N-CH3 + H2 O
CH3
A cetylcholine (ACh)
O
CH3
+
CH3 -C-O + HO-CH2 -CH2 -N-CH3
CH3
Choline
Acetate
 This rapid removal allows nerves to transmit more than
100 signals per second.
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Chapter 18, Section 6
Acetylcholine
Acetylcholine is linked to Alzheimer’s disease
 In Alzheimer’s disease, ACh levels may decrease by
90%.
 Aricept, an Alzheimer’s medication, slows the
breakdown of ACh in order to elevate the acetylcholine
levels in the brain.
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Chapter 18, Section 6
12
Catecholamines
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 Include dopamine,
norepinephrine, and epinephrine
 All of these are closely related in
structure and all are synthesized
from the amino acid tyrosine.
Chapter 18, Section 6
13
Dopamine
 produced in the nerve cells of the midbrain and acts as a
natural stimulant to give us energy and feelings of enjoyment.
 Control muscle movement, improve, cognition, memory, &
learning
 Cocaine and amphetamine block the reuptake of dopamine
resulting in a longer lifetime in synapse.
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Norepinephrine and Epinephrine
 Norepinephrine (noradrenaline) and epinephrine
(adrenaline) are hormonal neurotransmitters that play a
role in sleep, attention and focus, and alertness.
 Epinephrine is synthesized from Norepinephrine.
 Both are normally produced in the adrenal glands.
 Both are highly produced during the fight-or-flight
response, increasing blood pressure, heartv rate,
constrict blood vessel, dilate airways, stimulating
breakdown of glycogen.
 Administered during cardiac arrest.broncodialator.
 Low level leads to Attention Deficit Disorder(ADD)
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Serotonin (5-Hydroxytryptamine)
 Serotonin helps us relax, sleep deeply and
peacefully, think rationally, and it provides us a
feeling of well-being and calmness.
 Serotonin is synthesized from the amino acid
tryptophan.
 Psychedelic drugs stimulate the action of serotonin at
its receptors.
 Low serotonin levels may be associated with
depression, anxiety disorders, etc.
 Prozac and Paxil (antidepressant drugs) are selective
serotonin reuptake inhibitors (SSRIs).
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Prozac and Paxil
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Histamine
 synthesized in the nerve cells in the hypothalamus
from the amino acid, histidine.
 is produced by the immune system in response to
pathogens and invaders, or injury.
 When it combines with histamine receptors, it
produces allergic reactions – inflammation, watery
eyes, itchy skin,.etc.
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Amino Acid Neurotransmitters
Glutamate
 the most abundant neurotransmitters in the nervous system.
 stimulates the synthesis of nitrogen monoxide (NO).
Both Glutamate & NO involved in learning and memory
 Too much glutamate in the spinal cord causes degeneration
of nerve cells (Lou Gehrig’s disease of Muscular atrophy).
 Too rapid uptake of glutamate may results in schizophrenia:
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Gamma(g)-Aminobutyric Acid (GABA)
 the most common inhibitory neurotransmitter in the brain.
 produces a calming effect by inhibiting the ability of nerve
cells to send electrical signals to nearby nerve cells.
 Alcohol, & sedatives increases the inhibitory effects.
 Caffeine decreases the GABA levels in the synapses
causing opposite effects.
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© 2013 Pearson Education, Inc.
Chapter 18, Section 6